Continuously monitor the postfix Mailqueue (real time)

Ah, there's a flag for this.

instead of using tail -f /var/log/file we should be using tail -F /var/log/file

tail -F translates to tail --follow=name --retry as in;

• --follow=name: follow the name of the file instead of the file descriptor
• --retry: if the file is inaccessible, try again later instead of dying

You can use a combination of JMXTrans and Graphite. JMXTrans will periodically get the values of your JMX counters and then store them in Graphite. Graphite can then graph them for you.

As others have mentioned there are some SAAS solutions as well:

• https://metrics.librato.com/
• http://www.datadoghq.com